Metakaolin:
The
Pozzolan for Decorative Concrete
by
Michael Chusid
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You may never have heard of metakaolin, but
chances are it's coming your way. This admixture
- now being used in everything from fashionable
floors to custom countertops - makes concrete
stronger, brighter and more workable while decreasing
the amount of portland cement needed in the mix.
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Metakaolin is a type of pozzolan, materials that
enhance the strength, density and durability of concrete.
While decorative concrete contractors are at the
vanguard of many concrete innovations, some have
been slow to discover these materials. This is changing,
however, since high reactivity metakaolin, a relatively
new type Of pozzolan, offers features unavailable
from other highly reactive pozzolans; - improved
concrete appearance and workability.
Over half of the portland
cement concrete used in the United States now contains
pozzolans, according
to the National Ready Mix Concrete Association. Fly
ash and silica fume, the most common pozzolans, are
problematic in decorative concrete: They are typically
dark in color and, because they are industrial by
products, are not uniform in color. Silica fume is
a highly reactive pozzolan capable of producing very
strong and durable concrete, but results in "sticky" mixtures
that are difficult to place and finish and require
special curing techniques.
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| Andrew Goss crafts jewelry, such as this bracelet,
with metakaolin-enriched concrete. |
High-reactivity
metakaolin (HRM), on the other hand, is a white
powder that
brightens concrete. It is
manufactured especially for use in concrete and
to assure consistent appearance. It is highly reactive
and meets or exceeds the performance of silica
fume.
But unlike silica fume, it creates a creamy cement
paste that is easier to pump, trowels "like
butter," and can be cured with ordinary curing
techniques. Metakaolin is produced from kaolinite, a mineral
found in kaolin clay (also known as china clay).
The material is purified to remove compounds that
could discolor concrete, and then calcined under
controlled heat to create an amorphous aluminosilicate
that is reactive in concrete. Generally used to replace
5 percent to 20 percent of the portland cement in
a mixture, metakaolin also reduces emission of greenhouse
gases associated with portland cement manufacturing.
A growing number of concrete producers have metakaolin
on hand, and it is carried by distributors selling
plastering materials to swimming pool contractors.
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